This invention relates to flow switches and more particularly to a flow switch having a pressure sensing negative membrane.
In accordance with the prior art, a multiple piece construction design is used to sense and convert fluid pressure into mechanical action. Such an arrangement includes a diaphragm, a belleville spring, a mechanical pressure plate actuator, an O-ring seal and a retainer to contain the diaphragm seal. This type flow switch (Series 158F) is available from ITT Neo-Dyn (Chatsworth, Cal.). The differential pressure is generated by flowing fluid or air through a calibrated orifice or a venturi. The flow and differential pressure unit, is designed to divide the high and low pressure by clamp sealing the diaphragm by means of an elastomer O-ring seal, using a bolted or threaded retainer.
As pressure is applied to the positive rate diaphragm it immediately begins to deflect and, as pressure is increased to the diaphragm, deflection increases directly proportional to the applied pressure, until it is supported by the pressure plate and belleville. From this point, the load is transmitted thru the diaphragm to the pressure plate and belleville spring. The operation of the prior art unit in sequence is as the differential pressure is applied to the diaphragm it in-turn transmits force to the belleville spring and pressure plate assembly.
The actuator of the pressure plate presses against the hinge-arm of a hermetic or pseudo hermetic electrical assembly, and the hinge-arm applies force to a microswitch plunger causing electrical circuit transfer, from normally closed to open circuit (on increasing pressure), and normally open to closed circuit (on decreasing pressure).
The motion required to convert fluid pressure to mechanical work, results in belleville spring bending, sliding, scraping and plowing friction at the inside (hole) diameter, and at the outside diameter of the belleville spring. The resultant friction increases hysteresis and causes an increase in the unit deadband. The deadband is the difference between actuation and deactuation, or it can be explained as the difference between the point of operation versus the point that it returns to its pre-operated state.
The limitations of the prior art design are overcome by the invention for a flow switch with singe piece design wherein direct pressure sensing is achieved through a negative rate membrane. No seals are required for the sensing membrane which rides on a fluid or air bearing. The invention provides high vibration and shock resistance due to low moving mass design and provides maximum sensing area for any given package size. The simplified construction reduces weight while increasing reliability and the number unit life cycles.